Thermophotovoltaic System

This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 63/339,340 filed on May 6, 2022 and entitled “Thermophotovoltaic system without a vacuum or air gap,” and to U.S. Provisional Patent Application Ser. No. 63/440,898 filed on Jan. 24, 2023 and entitled “Thermophotovoltaic system without a vacuum or air gap,” both of which applications are expressly incorporated herein by reference in their entirety.

More than 60% of total world energy consumption every year ends up as waste heat. In the US only, waste heat originating from commercial, residential, and industrial sectors, amounts to 65 Quads, which is about equal to the energy contained in 65 trillion cubic feet of natural gas, 2925 million tons of coal, or 11 trillion barrels of crude oil. Such a vast renewable energy source has been largely untapped due to the lack of effective energy conversion technology that could capture and convert the waste heat into usable electrical energy. Limited technologies existing to harvest waste heat include thermoelectrical generator and thermophotovoltaics. However, there is no wide deployment so far for industrial and domestic applications based on these technologies because of their low energy conversion efficiency, high module cost (not scalable), and less power generation stability.

The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.

Disclosed embodiments include a thermophotovoltaic device comprising an emitter for emitting photons towards a receiver. The thermophotovoltaic device comprises an intermediate layer comprising a thermal insulating material with a low thermal conductivity of at most 1.4 W/m-K. The intermediate layer is positioned between the emitter and the receiver. The receiver comprising a photovoltaic cell configured to convert at least a portion of the photons into electric energy.

Additionally disclosed embodiments include a method for fabricating a thermophotovoltaic device with a zero-gap intermediate layer. The method comprises positioning an emitter for emitting photons towards a receiver. The method also comprises coupling an intermediate layer between the emitter and the receiver. The intermediate layer comprises a thermal insulating material with a thermal conductivity of at most 1.4 W/m-K. Further, the method comprises positioning the receiver comprising a photovoltaic cell configured to convert at least a portion of the photons into electric energy.

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

Additional features and advantages will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the teachings herein. Features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. Features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.

“Zero-gap Thermophotovoltaics (zero-gap TPV),” can address the long-standing challenges in waste heat harvesting by employing a fundamentally different physical principle from the existing technologies. Using zero-gap TPV disclosed embodiments eliminate the need of a vacuum or air gap in traditional gap-based TPVs, which is an obstacle that impedes the proliferation of TPVs and limited the large-scale cost-effective manufacturing. In at least one embodiment, an infrared-transparent intermediate layer is inserted between the thermal emitter and PV receiver as a functional component in the zero-gap TPVs.

illustrates an embodiment of a zero-gap TPV module. The depicted zero-gap TPV modulecomprise an emitterfor emitting photons towards a receiver. An intermediate layeris positioned between the emitterand the receiver. The receivercomprises a photovoltaic cell configured to convert at least a portion of the photons into electric energy. The depicted photovoltaic cellcomprises a P region, a Depletion region, and a N region. The receivermay also comprise gold reflector layer.

In at least one embodiment, the intermediate layermay comprise a thermal insulating material with a low thermal conductivity of at most 1.4 W/m-K, the intermediate layerpositioned between the emitter and the receiver. Additionally or alternatively, the intermediate layerfurther comprises a reflective index of at least 1.4. The intermediate layermay comprise a substantially infrared and visible spectrum-transparent material. For example, the intermediate layercomprises glass.

In at least one embodiment, the intermediate layercomprises a transparency to the wavelength range of 0.5-2.5 μm. Additionally or alternatively, the intermediate layercomprises ability to withstand high temperatures (>1000 degree Celsius). Further, the intermediate layermay comprise a low thermal conductivity (as low as possible, for example glass has 1.4 W/m·K of thermal conductivity). The intermediate layermay also comprise a high refractive index (as high as possible, glass is ˜1.5, GaAs is ˜3.3). Example materials for the intermediate layermay comprise glass, GaAS, Si, InP, CdTe, CdS, or some other similar material. High-K propagating wavesand low-K propagating wavesmay travel through the intermediate layer.

This new multilayer structure, serving as an effective IR waveguide, supports a much higher energy conversion efficiency and stable high-power generation. By replacing the vacuum or air gap with a solid-state layer, zero-gap TPV modulecan be fabricated as a whole and easily scaled up without the difficulties of costly hermetic seal and packaging. This can bring down the single module cost significantly, as well as enable large system level installation and integration.

The intermediate layerinserted between the thermal emitterand PV receiver, replacing the vacuum or air gap, serves as a lossless waveguide for frustrated infrared photons. The loss effects, potentially from the material type, geometry, and optical transmission, could significantly affect the energy conversion performance. A zero-gap TPV moduleis inherently a multi-layer optical structure that functions as a power generator. Interfaces between the layers within the zero-gap TPV determine at least some of the optical properties, and thus the energy conversion efficiency.

In order to calculate the power generation in a zero-gap TPV module, the first step is calculating the thermal radiation that reaches to the receiverfrom emitter. Net radiation between emitterand receiveris obtained by solving Maxwell's equations, and then applying fluctuation-dissipation theorem to them. One of the methods that is useful for solving Maxwell's equations is using dyadic Green's functions. In this regard, because of the existence of several layers, the thermal radiation can be obtained by solving dyadic Green's functions in a multilayered structure by using scattering matrix method. Thus, based on this method, the radiation heat flux between emitter layer, with a temperature of Tand a thickness of Z-Z, and receiver layer with a temperature of Tat a location of Zin a multilayered structure can be obtained by the following equation:

In order to predict the power generation in the PV cell, the carrier transport in a P-N junctionis modeled by solving the steady-state continuity equation numerically with finite difference method as follows:

where Dis a diffusion coefficient of electrons and holes, nis electron and hole concentration, gis electron-hole pairs generation rate, and Tis lifetime of electron/hole. An is the difference between local carrier concentration (n) and equilibrium carrier concentration (n). The equilibrium carrier concentration (n) depends on the intrinsic carrier concentration (n) and can be calculated by n=n/N. The lifetime of electron/hole can be obtained by taking into account the photon recycling of radiative recombination lifetime φτ, non-radiative Auger recombination lifetime τ, and non-radiative Shockley-Read-Hall (SRH) recombination lifetime τas follows:

The electron-hole pairs generation rate (go) can be calculated by using the thermal radiation model as follows:

In order to solve the equation (2), four boundary conditions are needed. At the top and bottom sides of the PV cell, recombination of electron-hole pairs is happened. Therefore, by considering a recombination velocity in these surfaces, the following equation can be applied for the boundary conditions:

Moreover, at the top and bottom sides of the depletion region, it is assumed that all the generated electron-hole pairs are swept, so recombination does not happen at these two boundaries. The thickness of depletion region can be obtained by the following equation:

where, εrepresents the static relative permittivity, Vrepresents the equilibrium voltage of the p-n junction that can be calculated by V=(kT/e)ln(NN/n).

After solving the equation (2) and obtaining the carrier distribution in the PV cell, in order to predict the power generation in the PV cell, first the generated current should be obtained. The generated current generation is a summation of generated currents in the depletion, P and N regions. It is assumed that all the generated electron-hole pairs in the depletion region generate current, so the generated current in the depletion region can be calculated as follows:

A lot of electron-hole pairs are generated in the P and N regions. However, because of the recombination in these regions, these generated electron-hole pairs can generate current only if they reach to the edges of depletion region. In this regard, the generated currents in P and N region depend on gradient of carrier concentration in the edges of depletion region and can be calculated as follows:

Thus, the total generated photocurrent is obtained as follows:

When there is no radiation on the PV cell, there is a current in the P-N junction that is called dark current (J) because of applied voltage on the cell. When the photocurrent is generated in the PV cell due to the radiation, the generated photocurrent opposes the dark current. The dark current can be calculated by solving the equation (2) while the source term is zero (g=0). After obtaining the dark current, I-V curve of PV cell is obtained by using J(V)=J−J(V) and then the power generation can be calculated.

The power conversion efficiency of PV cell is defined as the ratio of power generation to the net radiation heat transfer form emitter to PV cell. However, the power conversion efficiency of the TPV system is defined as the ratio of power generation to the total heat transfer form emitter to PV cell which consists of radiation and conduction.

The optical properties of InGaSb, GaAs, ZrN, and Au is known to those having skill in the art. The Intrinsic Carrier Concentration of InGaSb is assumed to be 2.22×10cm. The electron and hole diffusion coefficients are assumed to be 35.2 cmsand 18.3 cmsrespectively. The surface recombination velocity at the top and bottom of the PV cell is 2×10and 0 m·s. The non-radiative recombination lifetime of electron and hole of InGaSb are 9.8 ns and 31.1 ns respectively. Moreover, the radiative recombination lifetime of electron and hole of InGaSb are 13 ns and 1.27 μs respectively.

draws a comparison between the performance of zero-gap TPV moduleand thermoelectric (TE) system. According to this figure, the heat transfer between high and low temperature sources is higher for the zero-gap TPV modulewhen the emitter temperature is higher than 1250 K. To identify the reason for this issue,shows the contribution of radiation and conduction in the heat transfer between hot and cold sides of the zero-gap TPV system. According to this figure, the radiation heat transfer becomes higher than conduction when the emitter temperature is higher than 1200 K. Thus, because of the higher radiation contribution in heat transfer, the heat transfer of Zero-gap TPV moduleis higher than TE systems in high temperature. Therefore, this issue is a good potential for higher power generation in zero-gap TPV. Moreover, beside the higher heat transfer in zero-gap TPV module, the power conversion efficiency of zero-gap TPV is significantly higher than TE module in high temperature. According to, the efficiency of zero-gap TPV and TE module show completely different trend by increasing the emitter temperature. For a zero-gap TPV module, the efficiency is enhanced noticeably by increasing the temperature. This can be attributed to the two different facts. First, due to the band gap of 0.56 eV, by increasing the emitter temperature, the emission spectrum has higher energy flux in frequencies of higher than bandgap, so the efficiency is increased by increasing the emitter temperature. Second, the conduction part of heat transfer between emitter and PV celldoes not generate power in zero-gap TPV module. According to, by increasing the emitter temperature, the ratio between radiation and conduction increases significantly, so it causes the system efficiency to be improved.

shows the heat conduction and radiation in the proposed combined heat and power generation system. According to this figure, using a TE moduleand side heat collectorincreases the heat conduction in the system significantly. When the length of TE moduleand side heat collectoris 7.5 cm, the radiation is higher than conduction for emitter temperature of 1300 K, while the conduction is always higher than radiation when the length of TE moduleis 9.5 cm. On the other hand, the radiation is the same for all the cases.

shows a chartof the power generation in the TE and the TPV and also the heat generation inside and on the bottom heat collector for three different lengths of TE module. According to this figure, the power generation with the TPV moduleis higher than TE modulefor approximately all the emitter temperatures and TE lengths. When the emittertemperature is lower than 1100 and the length of the TE moduleis 9.5 cm, the TE modulegenerates more power than TPV module. There are two reasons for generating more power by the TPV modulein comparison to the TE modules. First, the temperature difference between both sides of TE modulesis significantly lower than that of the TPV module, so it has a lower potential for power generation. Second, the power conversion efficiency of TPV moduleis higher than that of the TE module, so the TPV modulehas the more ability to generate power.

Moreover, according to, the side heat generation is always higher than bottom heat generation because there is a higher heat conductance between the hot surface and the side heat collector, so the heat is absorbed more by the side heat collector. According to this figure, by increasing the length of TE module, the side heat generation is increased while the bottom heat generation is decreased. This can be attributed to the fact that by increasing the length of TE module and side heat collector, the intermediate slab becomes cooler and cooler, so the lower heat conduction is absorbed by the bottom heat collector. So, the length of TE moduleand side heat collectorare good design parameters for adjusting the heat generation in the bottom heat collectorand side collectors. The difference between the side and the bottom heat generation is the side heat collectorgenerates a higher potential heat because the outlet water temperature of side heat collectoris 90° C. while the outlet water temperature of bottom heat collectoris a few degrees of Celsius higher than ambient temperature, so the side heat generation is higher potential heat source.

Furthermore, according to the, by increasing the emitter temperature, the difference between the bottom heat generation when the length of TE moduleis 7.5 cm and 9.5 cm becomes smaller and smaller. Moreover, the slope of increment of bottom heat generation is higher than that of the side heat generation. This can be attributed to the fact that by increasing the emittertemperature, the radiation that is absorbed by the PV cellis increased significantly. Thus, a major part of the heat that is absorbed by the bottom heat collectorcomes from radiation. In this regard, because the radiation is the same for different lengths of the TE moduleand side heat collector, the amount of bottom heat generation tends to go to the same value by increasing the emitter temperature. Moreover, because of the aforementioned reason, the bottom heat generation follows the trend of radiation heat flux by increasing the emitter temperature. Therefore, because the radiation, in comparison to the conduction, is increased more and more by increasing the emitter temperature, the bottom heat generation, in comparison to the side one, shows a rapid increment with increasing the temperature.

shows the efficiency of power generation and side heat generation for different lengths of the TE modulesand the side heat collectors. According to this figure, by increasing the length of the TE moduleand the side heat collectorfrom 0 to 9.5 cm, the power conversion efficiency is reduced. This can be attributed to the fact by increasing the length of the TE module, the TE power generation is increased, so the contribution of TE power generation is increased. Because the power conversion efficiency of the TE moduleis smaller than TPV module, it reduces the total power conversion efficiency of the system.

Furthermore, by increasing the emitter temperature, the difference between power conversion efficiency of system with different TE module lengths is increased. This can be attributed to the fact that by increasing the emitter temperature, the efficiency of the TPV moduleis increased significantly, so the gap between efficiency of TPV and TE is increased. Therefore, increasing the length of the TE modulesin higher emitter temperature, reduces the power conversion efficiency of the system more and more. Moreover, according to this figure, the power conversion efficiency is increased by increasing the emitter temperature. This can be attributed to the fact that by increasing the emitter temperature, the TPV power conversion efficiency is increased, so it causes the system to have higher efficiency. However, by increasing the emitter temperature, the contribution of side heat generation is decreased. This can be attributed to the fact that by increasing the emitter temperature, the radiation part of the incoming heat is increased. Because the radiation is absorbed by the PV cell, most of the radiation heat flux is absorbed by bottom heat collector. Thus, it causes the side heat collectorto absorb less part of incoming heat flux from the emitter when the emitter temperature is increased.

In at least one embodiment, the TPV modulecomprises several different interfaces between the different layers. For example, an emitter interface(shown in) is located between the intermediate layerand the emitter. Additionally, a receiver interfaceis located between the intermediate layerand the receiver. In at least one embodiment, the receiver interfacebetween the intermediate layerand the receivercomprises an optical epoxy. For example, when constructing the TPV modulethe manufacturer may bond the intermediate layerwith the receiverwithin an optical epoxy. The optical epoxy may appear substantially transparent to radiation travelling through the intermediate layer.

Additionally or alternatively, in at least one embodiment, the emitter interfacebetween the intermediate layerand the emittercomprises a surface with nanometer-scale roughness. The emitterand the intermediate layermay then be bonded through a pressure connection where the two components are pressed together along the nanometer-scale roughness surfaces. In at least one embodiment, a pressure of 20 to 60 psi is sufficient for bonding the emitterand the intermediate layer.

Additionally or alternatively, in at least one embodiment the emittercomprises a film that is directly applied to the intermediate layer. For example, the film may have adhered to the intermediate layersuch that no additional pressure or action is needed to form the desired bond.

Additionally, in at least one embodiment, the receiver interfacebetween the intermediate layerand the receivercomprises a first nano-pattern fabricated on a surface of the receiverand a second interconnecting nano-pattern on a surface of the intermediate layer. The second interconnecting non-pattern may comprise parallel lines that are configured to interconnect with parallel lines in the first nano-pattern. Various other patterns that provide some lateral friction may be utilized.

The following discussion now refers to a number of methods and method acts that may be performed. Although the method acts may be discussed in a certain order or illustrated in a flow chart as occurring in a particular order, no particular ordering is required unless specifically stated, or required because an act is dependent on another act being completed prior to the act being performed.

shows a flowchart in a methodfor fabricating a thermophotovoltaic device with a zero-gap intermediate layer. Methodincludes an actof positioning an emitter. Actcomprises positioning an emitterfor emitting photons towards a receiver. For example, during manufacture of the TPV module, the emitteris positioned on one side of an intermediate layerand the receiveris positioned on the other side.

Methodadditionally includes an actof coupling an intermediate layer. Actcomprises coupling an intermediate layerbetween the emitterand the receiver. The intermediate layercomprises a thermal insulating material with a thermal conductivity of at most 1.4 W/m-K. For example, the emittermay be coupled to the intermediate layerthrough nano-scale course surfaces that are pressed together. Further, the intermediate layermay be coupled to the receiverthrough an optical epoxy.

Methodalso includes an actof positioning the receiver. Actcomprises positioning the receiver. The receivercomprises a photovoltaic cellconfigured to convert at least a portion of the photons into electric energy.